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Treebeard's Stumper Answer
6 Dec 96

The days are getting noticeably shorter as we approach the Winter Solstice on December 21. It's been cold this week even though we've had bright sunny days. The morning sun shines on the frosty grass as we start our day at the Middle School. The frost soon disappears, but the chill remains throughout the afternoon. How can it be stay so cold despite the bright sun? Why does the winter sun provide so little heat?

The winter sun can be bright, but it's lower in the sky, and that's why it provides so little heat. Shine a flashlight straight down. It makes a small bright circle. Now tilt the flashlight. The circle of light spreads out into an ellipse. Because it is more spread out, the light (and heat) is less intense. The same amount of sunlight that we have in the summer is spread out over nearly twice the area in the winter, and it warms us only half as much. Winter days are chilly no matter how bright.

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Marc Kummel / mkummel@rain.org